Ruby- America (The cross for Christianity, and maybe, she's the youngest? Dunno)
Considering you did nothing but list continents here, I wonder if you mean the United States of Murica, or America the continent.
Though in both cases that line of thinking is full of flaws. The US is not a christian nation, it's not even half that religious. And in terms of America as a whole, there's a lot more than just Christianity. In fact, if we said Ruby was American just for that, we could say she's European too.
Weiss- Europe (She's elegant and her name's German)
Elegancy means nothing, every single culture that was above tribal stage, including ones outside Europe, had it's elegant elements that even today are still seen as elegant.
Blake- Africa (She's a Faunus and gets hate from some people, like the Africans used to be treated)
You're forgetting that she lived in Remnant's Australia.
Also, saying she represents Africa just because Faunus are Remnant's equivalent of black people, doesn't really have that much weight. Black people were not the only ones discriminated and for-some-time slaved.
I could say that Blake represents America, because the native americans were also enslaved and mistreated in the colonial times. Exact same logic.
Yang- Asia (Her name is Chinese)
That's the weakest spitballing in this entire thing. A chinese name means nothing when things like migration means that, for example, X chinese dude with Y chinese name has been Amurican all his life.
And Yang as a whole, in terms of attitude, personality and fashion sense, is one of the least asian characters in this entire show.